1. Field Of The Invention
The present invention relates to wastewater treatment methods and systems, and more particularly pertains to the removal of emulsified oil, heavy metals, and other contaminants from water and water based solutions. Such water based solutions find applications for metal parts cleaning in the metal working industry, cutting and grinding oils, die casting, metal plating, pressure washers, and steam cleaning. Another example application is in the field of painting, and particularly in spray booth applications. A further application is in the field of removing printer's ink from water based cleaning solutions resulting from the cleaning of printing equipment.
2. Description Of The Prior Art
Many techniques are available to help remove contaminants from wastewater, reduce or eliminate the hazardous nature of the effluent and prepare the water for release into the environment. Most conventional techniques are multi-step and require time as well as extensive handling to accomplish the task of removing suspended solids, oils, and metal ions from the water. Even if the problems of time and handling are alleviated, a significant problem remains because most available methods leave residues still classified as a hazardous waste. The currently accepted best available treatment for most of these residues is to stabilize them in a matrix that will not allow any leaching of the hazardous materials into the surrounding environment. This is, of course, an extra manipulation that requires time and adds even more cost to the operations of already overburdened treatment facilities.
Waste waters contain a mixture of suspended or colloidal solids, dissolved metals and ions, and organic contaminants. The art of waste water cleanup is predicated on the concept of precipitating these species out of solution into an aggregated mass that either floats on the surface where it can be skimmed off, or sinks to the bottom where it can be removed by filtration or decantation. The process of forming that solid is called flocculation, and the solid that forms is called a floc (actually short for floccule, although the term is no longer used). The mechanism of flocculation is rather complex, but it involves the presence of extremely tiny colloidal particles and dissolved ions. All particles exert forces, both attractive and repulsive, on each other. For colloidal particles, the attractive forces are much weaker than the repulsive forces; that fact, coupled with their extremely small size, allows them to remain suspended in solution. However, under the right conditions, the attractive forces can be strengthened and the repulsive forces shielded so that flocculation occurs.
The traditional approach to purifying wastewater containing particulates, oils, and dissolved metals involves flocculating these contaminants by successively adjusting the wastewater conditions to the point where each particular class of contaminant will become insoluble and agglomerate into a small mass that can be removed from the much larger volume of purified water. The first step usually consists of adjusting the water to acidic conditions to break any oil emulsions that may exist. Most of the oil will float to the top of the treatment tank where it can be removed by skimming. A cationic polymer (called a flocculent) is typically added at this point to attract negatively charged species in the water as well as any remaining oil. When the polymer performs properly, it flocculates into a mass that can be removed from the water by decantation and filtration. The water must be tested to be sure that enough oil has been removed to achieve appropriate standards. If it does not pass this test, the treatment is repeated until it does. This process is often time consuming because a certain amount of trial and error is frequently required to find the right polymer for generating a floc that is dense enough to separate the oil. Often the polymer/oil interaction is not particularly strong. Since oil and water don't mix whereas like materials readily solubilize each other, the oil tends to associate with the polymer chains because they are more similar to the oil than the water. However, since the polymer itself is polar enough to be soluble in water, it becomes only slightly more desirable as a partner for the oil than the water. If enough water washes through the polymer/oil floc, the two will separate. Such a floc is unlikely to pass the leachability tests for hazardous materials.
Additionally, prior art wastewater systems involve large treatment tanks and are suitable only for treating large and continuously supplied wastewater streams. Many industries generating relatively small quantities of wastewater must currently pay large fees for trucking to a treatment facility. The EPA and various state and local government agencies are adopting increasingly stringent standards for wastewater disposal. Environmental law now provides stiff economic penalties and cleanup cost liability on land owners where hazardous materials have been dumped. Accordingly, there is a need for and considerable interest in a wastewater treatment method and apparatus capable of economically treating wastewater on a relatively small batch basis which results in a landfill acceptable sludge. Further, many industrial water based solutions contain valuable chemical components which would result in a great savings if recovered and recycled during wastewater treatment.